1. Field of the Invention
The present invention relates to a semiconductor device having a high-dielectric-constant thin film and a manufacturing method thereof, and more particularly to an art making a gate insulating film that constitutes a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) attain higher performance and lower power consumption.
2. Description of the Related Art
As a material for the gate insulating film of the MOSFET, the silicon oxide film is in wide use having an excellent process stability and a high insulation capability. Meanwhile, along with the current miniaturization, the thickness of the gate insulating film has been decreasing, and in the device whose gate length is 100 nm or less, it has become essential under scaling low that the film thickness of the silicon oxide film which serves as a gate insulating film is 1.5 nm or less. Yet, when such an extra thin insulating film is utilized, the tunneling current through the insulating layer, in applying the gate bias, becomes substantial in comparison with the source/drain current, which leads to a serious adverse effects on attempts to attain higher performance and lower power consumption for the MOSFET.
Another problem to accompany the reduction of the thickness of the gate insulating film is the diffusion of the dopants from the gate electrode (the polysilicon electrode). The gate electrode used in the MOSFET, in general, acquires its metallic properties through the high concentration doping which is applied to the polysilicon grown on the gate insulating film, and, thus, when the silicon oxide film is very thin, there arises a problem that the dopants may diffuse from the gate electrode to the silicon substrate side, passing through the insulating film layer.
In order to solve the above problems of the increase in leakage current and the penetration of the dopants that are brought about by the reduction of the film thickness of the gate insulating film, various techniques have been being developed. One of such techniques is a method in which by adding nitrogen to the silicon oxide film, the dielectric constant of the film is made higher than that of the pure silicon oxide film and thereby the effective (electrical) film thickness of the gate insulating film is made lessened without reducing its physical film thickness. Since it was, furthermore, confirmed that the nitrogen addition also suppresses the diffusion of dopants within the insulating film, and, therefore, the technique of the nitrogen addition to the gate insulating film has been drawing considerable attention as a highly promising technique capable to solve the two problems mentioned above. However, this technique has, as pointed out, disadvantages that the nitrogen addition to the silicon oxide film can only raise its dielectric constant up to a certain value and besides nitrogen may cause the interfacial defects and the fixed charge generation in the film, resulting in the poor mobility as well as the lower reliability of the transistor.
In consequence, the investigations are under way to find a material having a dielectric constant higher than those of the silicon oxynitride film and the silicon nitride film as well as an preventive effect on the dopant diffusion to replace the silicon oxide film as the next generation material of the gate insulating film. Firstly, among materials having a high dielectric constant, Al2O3, ZrO2, HfO2, rare-earth element oxides such as Y2O3, and lanthanoid type rare-earth element oxides such as La2O3 and, in addition, thin films of silicate of these substances are being examined as candidates for such a material.
This is based on an idea that even if the gate length becomes minute, the use of a high-dielectric-constant film of this sort makes it possible to prevent the tunneling current well with a film thickness the scaling law allows to have, while maintaining the capacitance of the gate insulating film at the same time.
Now, for any type of the gate insulating film, the film thickness of the insulating layer obtained by the reverse calculation from the gate capacitance under the assumption that the material of the gate insulating film is a silicon oxide film is called the film thickness in terms of the silicon oxide film thickness. That is, when the dielectric constants of the insulating film and the silicon oxide film are taken to be ∈h and ∈o, respectively, and the thickness of the insulating film is taken to be dh, the film thickness in terms of the silicon oxide film thickness de becomes de=dh(∈o/∈h). This implies, in other words, that, with the material having a dielectric constant ∈h that is larger than ∈o, the insulating film with a certain thickness may become equivalent to a thin silicon oxide film. As the dielectric constant ∈o of the silicon oxide film is 3.9 or so, if a high-dielectric-constant film, for example, with ∈h=39 is used, the film 15 nm thick has a film thickness of 1.5 nm in terms of the silicon oxide film thickness so that this film can heavily reduce the tunneling current.
As described above, in developing the next generation MOSFET, various high-dielectric-constant films are, as the gate insulating film material, being examined for use, and the afore-mentioned metal oxide thin films and silicate thin films are regarded as strong candidates for the high-dielectric-constant film. Nevertheless, even these high-dielectric-constant films have been shown to have the following problems.
Firstly, the thermal stability of the high-dielectric-constant film is itself a problem. Namely, it has been reported that, in the step of conducting the heat treatment to activate dopants implanted in the gate electrode, the afore-mentioned gate material with a high dielectric constant becomes crystallized or the interfacial reaction with the silicon substrate proceeds. When crystallization of the high-dielectric-constant film takes place, with the boundary (grain boundary) appearing among grains, the insulation characteristics on these grain boundaries deteriorate and the film thickness within a plane becomes non-uniform due to crystallization is brought about. As the means of overcoming this problem of crystallization, it is effective to select a high-dielectric-constant material with a high thermal stability, in the first place, and besides apply the nitrogen addition to the metal oxide or silicate film. Meanwhile, oxygen in the vapor phase readily diffuses into the high-dielectric-constant film so that a reactive layer may be disadvantageously formed on the interface with the silicon substrate at the time of the film growth and the subsequent heat treatment.
In regard to this problem, a structure in which a very thin (normally 0.5 nm to 1 nm or so thick) silicon oxide film is inserted on the interface between a high-dielectric-constant film and a silicon substrate is being examined. Moreover, a recent report indicated that a silicon oxynitride film can be used as the afore-mentioned interfacial insertion layer with effect.
As the second problem for the high-dielectric-constant gate insulating film, similar to the silicon oxide film, deterioration of the device characteristics thereof due to the dopant penetration is well known. While seriousness of this problem varies with the material type of the insulating film, and, moreover, the physical film thickness for the high-dielectric-constant gate insulating film can be set thicker than that for the silicon oxide film, if the diffusion rate of the dopants in the film is high and a polysilicon gate electrode or polysilicon germanium electrode is utilized, this problem becomes a fatal one. Nevertheless, the recent investigation demonstrated that nitrogen addition to Al2O3 or ZrO2 can suppress the dopant diffusion well.
For the third problem, there is pointed out deterioration of electric characteristics of the interface between a high-dielectric-constant thin film and a silicon substrate. Compared with the interface of the conventional silicon oxide film, the interface of the high-dielectric-constant thin film has a higher interfacial defect density of, in general, not less than 1011/cm2, which is liable to cause the following problems. These interfacial defects (or defects within the film) worsen the MOSFET mobility in such a way that it may become even less than a quarter of that with the silicon oxide film. Further, the presence of fixed charges in the film and on its interface disadvantageously varies the threshold of the transistor operations. For a remedy of these problems, like the remedy of the first problem, the insertion of a silicon oxide film is effective. However, if the interfacial silicon oxide film layer is thick, the film thickness of the whole gate insulating film in terms of the silicon oxide film thickness increases a great deal. On the other hand, if the interfacial oxide film layer is thin, satisfactory interfacial thermal stability or sufficient preventive effects on the dopant penetration cannot be obtained. Furthermore, although the structure in which an extra thin silicon oxynitride film or silicon nitride film is inserted on the interface between a high-dielectric-constant film and a silicon substrate is effective for the improvement on the interfacial thermal stability and suppression of the dopant penetration, deterioration of electric characteristics remains. This can be attributed to the interfacial defects caused by the additional presence of nitrogen and, in comparison with the conventional interface of the silicon/silicon oxide films, more deterioration of the mobility and reliability is brought about.
As described above, although the first and the second problems can be successfully overcome by making nitrogen contained in the high-dielectric-constant gate insulating film, once inside, the presence of the nitrogen on the interface with the silicon substrate has adverse effects. In making nitrogen contained in the silicon oxide film, if the sample is subjected to a heat treatment in the atmosphere of a nitrogen containing gas such as NH3 or NO gas, nitrogen can be introduced into the film, but a large amount of segregated nitrogen tends to be left on the interface with the silicon substrate, which causes the lowering of the mobility and the deterioration of the reliability as described for the third problem. Further, for the high-dielectric-constant insulating film, too, nitrogen can be introduced into the film by annealing that in the atmosphere of a nitrogen containing gas, but, in this case, too, a possibility of a similar problem of nitrogen segregation on the interface with the silicon substrate cannot be ruled out.
As another method of introducing nitrogen into a high-dielectric-constant thin film, there is proposed to use the step of applying an oxidation treatment to a metal nitride film (Koyama et al., Tech. Dig. IEDM 2001, p. 459). In short, a ZrN film is grown on the surface of a silicon substrate by the sputtering method, and by applying an oxidation treatment thereto at 500° C., nitrogen is added into ZrO2 and thereby a better thermal stability than the conventional ZrO2 film can be provided. However, in this method, when ZrN is grown as well as when the oxidation treatment is conducted, a SiON layer containing nitrogen at a high concentration is formed on the interface with the silicon substrate so that this method either cannot bring a thorough solution for the third problem described above.